U.S. patent number 6,664,931 [Application Number 10/201,015] was granted by the patent office on 2003-12-16 for multi-frequency slot antenna apparatus.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Henry H. Nguyen, Michael S. Pieper.
United States Patent |
6,664,931 |
Nguyen , et al. |
December 16, 2003 |
Multi-frequency slot antenna apparatus
Abstract
A multi-frequency antenna apparatus includes an open-ended slot
antenna (20) operable at high frequencies, cascaded in series with
a antenna element (22) operable at low frequencies, results in two
distinct antennas fed by a single excitation port (33). A U-shaped
conductive strip (21) defining the slot antenna (20) has a ground
connection (37) at one end near the open-ended side of the slot
antenna (20) and a virtual feed point (30) on the other end coupled
to the antenna element (22). The electrical length from the ground
connection (37) to the feed point is about 1/4 of a wavelength of
the first frequency creating a virtual open at the feed point
(30).
Inventors: |
Nguyen; Henry H. (Fort Worth,
TX), Pieper; Michael S. (Fort Worth, TX) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
|
Family
ID: |
27788727 |
Appl.
No.: |
10/201,015 |
Filed: |
July 23, 2002 |
Current U.S.
Class: |
343/767;
343/700MS |
Current CPC
Class: |
H01Q
1/243 (20130101); H01Q 1/38 (20130101); H01Q
13/10 (20130101); H01Q 5/357 (20150115); H01Q
5/40 (20150115) |
Current International
Class: |
H01Q
1/38 (20060101); H01Q 13/10 (20060101); H01Q
1/24 (20060101); H01Q 5/00 (20060101); H01Q
013/10 () |
Field of
Search: |
;343/767,7MS,702,768,769,770,846 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Clinger; James
Attorney, Agent or Firm: Mancini; Brian M.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to U.S. patent application, Ser. No.
9/408,672 by inventors Nguyen, Kwan and Pieper. The related
application is assigned to the assignee of the present application,
and are hereby incorporated herein in its entirety by this
reference thereto.
Claims
What is claimed is:
1. An antenna apparatus operable at a first frequency and a second
frequency, the antenna apparatus comprising: a dielectric material;
a slot antenna defined a conductive strip disposed on a top surface
of a first portion of the dielectric material to form a
substantially rectangular slot, the slot antenna being operable at
a first frequency with the slot having an electrical length of
about 1/8 of a wavelength of the first frequency; and an antenna
element disposed on a second portion of the dielectric material,
the antenna element is operable at a second frequency; and the
conductive strip having a ground connection at one end of the slot
antenna and near an opposite end of the conductive strip having a
feed point coupled to the antenna element, wherein the electrical
length from the ground connection to the feed point is about 1/4 of
a wavelength of the first frequency.
2. An antenna apparatus according to claim 1, wherein a portion of
the antenna element is arranged in a coil configuration.
3. An antenna apparatus according to claim 1, wherein the coil
configuration consists of a conductive strip wrapped around the
second portion of the dielectric material.
4. An antenna apparatus according to claim 1, wherein the coil
configuration consists of two sets of conductive strips disposed on
opposing surfaces of the second portion of the dielectric material
and connected together through the dielectric material by vias to
form a coil winding.
5. An antenna apparatus according to claim 1, further comprising a
microstrip feed-line disposed on a bottom surface of the dielectric
material, opposing and below the slot antenna and being
perpendicular to a long axis thereof.
6. An antenna apparatus according to claim 5, wherein the
microstrip feed-line couples energy to the slot antenna near the
open-end thereof.
7. An antenna apparatus according to claim 5, wherein the
microstrip feed-line includes a tuning portion extending parallel
to the long axis of the slot antenna.
8. An antenna apparatus operable at a first frequency and a second
frequency, the antenna apparatus comprising: a dielectric material;
an open-ended slot antenna defined by a U-shaped conductive strip
disposed on a top surface of a first portion of a dielectric
material to form a substantially rectangular slot, the slot antenna
being operable at a first frequency with the slot having an
electrical length of about 1/8 of a wavelength of the first
frequency; an antenna element disposed on a second portion of the
dielectric material, the antenna element is operable at a second
frequency, and the conductive strip having a ground connection at
one end and near an opposite end of the conductive strip having a
feed point coupled to the antenna element, wherein the electrical
length from the ground connection to the feed point is about 1/4 of
a wavelength of the first frequency, the ground connection and feed
point being located near an open-end of the slot antenna; and an
excitation port electromagnetically coupled to both the slot
antenna and the antenna element.
9. An antenna apparatus according to claim 8, wherein a portion of
the antenna element is arranged in a coil configuration.
10. An antenna apparatus according to claim 8, wherein the coil
configuration consists of a conductive strip wrapped around the
second portion of the dielectric material.
11. An antenna apparatus according to claim 8, wherein the coil
configuration consists of two sets of conductive strips disposed on
opposing surfaces of the second portion of the dielectric material
and connected together through the dielectric material by vias to
form a coil winding.
12. An antenna apparatus according to claim 8, further comprising a
microstrip feed-line disposed on a bottom surface of the dielectric
material, opposing and crossing below the slot antenna and being
perpendicular to a long axis thereof.
13. An antenna apparatus according to claim 12, wherein the
microstrip feed-line couples energy to the slot antenna near the
open-end thereof.
14. An antenna apparatus according to claim 12, wherein the
microstrip feed-line includes a tuning portion extending at least
partially parallel to the long axis of the slot antenna, the
microstrip feed-line having a shape selected from one of the group
of a "L" shape, a "C" shape, and a "T" shape.
15. A multi-band radiotelephone including an antenna apparatus
operable at a first frequency and a second frequency, the antenna
apparatus comprising: a dielectric material; an open-ended slot
antenna defined by a U-shaped conductive strip disposed on a top
surface of a first portion of a dielectric material to form a
substantially rectangular slot, the slot antenna being operable at
a first frequency with the slot having an electrical length of
about 1/8 of a wavelength of the first frequency; a coil antenna
element disposed on a second portion of the dielectric material,
the antenna element is operable at a second frequency, and the
conductive strip having a ground connection at one end and near an
opposite end of the conductive strip having a feed point coupled to
the antenna element, wherein the electrical length from the ground
connection to the feed point is about 1/4 of a wavelength of the
first frequency, the ground connection and feed point being located
near an open-end of the slot antenna; and an excitation port
electromagnetically coupled to both the open-ended slot antenna and
the coil antenna element.
16. An antenna apparatus according to claim 15, wherein the coil
configuration consists of two sets of conductive strips disposed on
opposing surfaces of the second portion of the dielectric material
and connected together through the dielectric material by vias to
form a coil winding.
17. An antenna apparatus according to claim 15, wherein the
excitation port includes a microstrip feed-line disposed on a
bottom surface of the dielectric material, opposing and below the
slot antenna and perpendicular to a long axis thereof.
18. An antenna apparatus according to claim 17, wherein the
microstrip feed-line couples energy to the slot antenna near the
open-end thereof.
19. An antenna apparatus according to claim 17, wherein the
microstrip feed-line includes a tuning portion extending at least
partially parallel to the long axis of the slot antenna.
20. An antenna apparatus according to claim 17, further comprising
additional conductors disposed on the bottom surface of the
dielectric material, the additional conductors being coupled across
the slot, thereby causing the antenna to be radiant at more than
one frequency band.
Description
FIELD OF THE INVENTION
This invention relates generally to antennas, and more particularly
to multi-frequency antennas including a slot antenna.
BACKGROUND OF THE INVENTION
Wireless communications technology today requires cellular
radiotelephone products that have the capability of operating in
multiple frequency bands. The normal operating frequency bands, in
the United States for example, are analog, Code Division Multiple
Access (CDMA) or Time Division Multiple Access (TDMA) at 800 MHz,
Global Positioning System (GPS) at 1500 MHz, Personal Communication
System (PCS) at 1900 MHz and Bluetooth.TM. at 2400 MHz. Whereas in
Europe, the normal operating frequency bands are Global System for
Mobile Communications (GSM) at 900 MHz, GPS at 1500 MHz, Digital
Communication System (DCS) at 1800 MHz and Bluetooth.TM. at 2400
MHz. The capability to operate on these multiple frequency bands
requires an antenna structure able to handle all these
frequencies.
External antenna structures, such as retractable and fixed "stubby"
antennas have been used with multiple antenna elements to cover the
frequency bands of interest. However, these antennas, by their very
nature of extending outside of the radiotelephone and of having a
fragile construction, are prone to damage. In particular, as the
size of radiotelephones shrink, users are more likely to place the
phone in pockets or purses where they are subject to jostling and
flexing forces that can damage the antenna. Moreover, retractable
antennas are less efficient in some frequency bands when retracted,
and users are not likely to always extend the antenna in use since
this requires extra effort. Further, marketing studies also reveal
that users today prefer internal antennas to external antennas.
The trend is for radiotelephones to incorporate fixed antennas
contained internally within the radiotelephone. However, this
typically increases the size of the radio telephone to accommodate
the antenna structure, and it is difficult to maintain antenna
efficiency, since the antenna element are now placed in proximity
to other conductive components in the radiotelephone. Moreover, the
antenna is more susceptible to interference from these same
conductive components, further impairing efficiency, particularly
in the low frequency bands.
Slot and microstrip transmission line antennas can be used in high
frequency applications and have a very low profile. However, due to
size constraints, these antennas can only operate in one single
frequency band. Slot antennas can be implemented with cutout in a
metal surface. Prior art resonant slot antenna geometries include a
half wavelength (.lambda./2) full slot antenna where both ends of
the slot are closed, and the length of the slot is a half
wavelength (about 80 mm at 1800/1900 MHz, which is quite long and
not practical for cellular phone). Another type of slot antenna is
a one-quarter wavelength (.lambda./4) open-end slot antenna 10 as
shown in prior art FIG. 1. For a .lambda./4 slot antenna 10, the
length 12 of the slot 14 is a quarter wavelength with one end of
the slot 14 closed while the other end is open. The slot 14 is
excited differentially by energy coupled from an excitation port
providing a positive charge 13 and a negative charge 15 near the
closed end of the slot 14 and perpendicular to the slot as shown.
The excitation port is typically provided by a microstrip line
embedded under the slot. A conductive ground plane 16 surrounds the
slot 14. More than one slot antenna can be used in a radiotelephone
to obtain radiation in multiple frequency bands. However, separate
antennas require separate excitation ports and individual
electronic tuning mechanisms, which increases size and cost.
Therefore, there is a need for a small size and low cost internal
antenna apparatus with and multi-band frequency radiation
capability. Another desired advantage would be to provide
performance comparable to external multi-band antennas. It would
also be of benefit to provide this antenna apparatus driven by a
single excitation port.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a top plan view of a prior art quarter-wavelength slot
antenna;
FIG. 2 shows a perspective view of the antenna apparatus of the
present invention according to a first preferred embodiment;
FIG. 3 shows a top plan view of the antenna apparatus of the
present invention according to an alternate first preferred
embodiment
FIG. 4 shows a top plan view of the antenna apparatus of the
present invention according to a second preferred embodiment;
and
FIG. 5 shows a cross-sectional view of the antenna apparatus of
FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides an internal antenna apparatus with
multi-band frequency radiation capability. In particular, a coil
antenna is coupled to, and excited by, an open-end slot antenna on
a common substrate to cover two distinct frequency bands. The
structure described in the present invention provides a compact,
low-profile antenna apparatus that can be mounted internally in a
radiotelephone with performance comparable to external multi-band
antennas. Moreover, the configuration of the open-end slot antenna
driving the coil antenna allows this antenna apparatus to be driven
by a single excitation port.
The invention will have application apart from the preferred
embodiments described herein, and the description is provided
merely to illustrate and describe the invention and it should in no
way be taken as limiting of the invention. While the specification
concludes with claims defining the features of the invention that
are regarded as novel, it is believed that the invention will be
better understood from a consideration of the following description
in conjunction with the drawing figures, in which like reference
numerals are carried forward. As defined in the invention, a
radiotelephone is a portable or mobile communication device that
communicates information to a base station using electromagnetic
waves in the radio frequency range.
The concept of the present invention can be advantageously used on
any electronic product requiring the transceiving of RF signals.
Preferably, the radiotelephone portion of the communication device
is a cellular radiotelephone adapted for personal communication,
but may also be a pager, cordless radiotelephone, or a personal
communication service (PCS) radiotelephone. The radiotelephone
portion may be constructed in accordance with an analog
communication standard or a digital communication standard. The
radiotelephone portion generally includes a radio frequency (RF)
transmitter, a RF receiver, a controller, an antenna, a battery, a
duplex filter, a frequency synthesizer, a signal processor, and a
user interface including at least one of a keypad, display, control
switches, and a microphone. The radiotelephone portion can also
include a paging receiver. The electronics incorporated into a
cellular phone, two-way radio or selective radio receiver, such as
a pager, are well known in the art, and can be incorporated into
the communication device of the present invention.
FIGS. 2 and 3 show an antenna apparatus operable at a first
frequency (high frequency band) and a second frequency (low
frequency band), in accordance with the present invention. An
open-ended slot antenna 20 resonant at the first frequency is
connected to an antenna element 22 resonant at the second
frequency, cascaded in series. The slot antenna 20 feeds the
antenna element 22. Preferably, a portion 24 of the antenna element
is arranged in a coil configuration. This is done to reduce the
overall length of the antenna structure. However, it should be
recognized that the antenna element could also be a straight wire
or other arrangement. The slot antenna can also be close-ended on
both ends, but this increases the size of the antenna structure. It
should be recognized that the thickness of the dielectric as shown
is exaggerated to reduce visual clutter.
In particular, a conductive strip 21, a quarter-wavelength long at
the first frequency and folded in a U-shape, is disposed on a first
portion of a dielectric material 23 and a slot 29 is implemented in
the conductive strip 21 to define an open-ended slot antenna 20.
The dielectric substrate is rectangular having two long sides and
two short sides and opposing top and bottom major surfaces. The
U-shaped conductive strip is disposed on the top surface of the
dielectric substrate 23. The U-shaped conductive strip includes two
side members defining each leg of the U-shape, each about
one-eighth the predetermined wavelength in length, and an end
member connecting the legs or side-members to complete the U-shape.
The side and end members define a substantially rectangular slot 29
extending substantially parallel to the long sides. The slot 29 is
closed at a first end (closed end) by the end member, and open at a
second end (open end). The antenna further comprises a microstrip
feed line 33 attached to the bottom surface of the dielectric
substrate for electromagnetically coupling an RF signal between the
antenna and an RF device, such as a radiotelephone for example. The
microstrip feed line extends across and perpendicular to the slot
proximate the second end of the slot, and further extending across
a portion of the two side members. A ground point 37 is
electrically coupled to a first one of the two side members of the
U-shaped conductive strip 21 and positioned proximate the second
end of the slot 29.
The length, width and location of the slot affects the operating
frequency band achieved. The slot antenna 20 is operable at the
first frequency, and has an electrical length 27 of about 1/8 of a
wavelength of the first frequency. The conductive strip 21 has a
ground connection 37 at one end near the open-ended side of the
slot antenna 20, and the antenna element 22 is connected to the
opposite end of the conductive strip 21 at a virtual feed point 30.
The electrical length 28 from the ground connection 37 to the
virtual feed point 30 is about 1/4 wavelength at the first
frequency. This quarter-wavelength conductive strip serves two
significant purposes at the first frequency: a) maximizing the
potential difference across the slot 29 at the open-end to achieve
maximum radiation from the slot antenna 20, and b) creating an open
circuit at the virtual feed point 30 such that the addition of the
antenna element 22 at the virtual feed point 30 will not
electrically affect the slot antenna 20. A virtual feed point can
also be used for a closed-ended slot antenna. along the same
principles.
The antenna element 22 is disposed on a second portion of the
dielectric material 23, and is operable at the second frequency.
The coil portion 24 can include one or both of: a) a conductive
strip 25 that is wrapped around the sides of the second portion of
the dielectric material 23 (as shown in FIG. 2), or b) two sets of
substantially parallel conductive strips disposed on opposing
surfaces of the second portion of the dielectric material 23 and
connected together through the dielectric material by vias 26 to
form a coil winding (as shown in FIG. 3). However, it is envisioned
that either one or the other technique would be used in practice.
It is preferred that all vias are used inasmuch as this is more
easily accomplished in the manufacture of the antenna structure.
For example, vias can be formed by plating through-holes in the
dielectric sheets after sintering, or filling with conductive
materials such as conductive cement or epoxy.
The present invention includes a single excitation port 33 and a
microstrip feed-line portion 34 disposed on the dielectric material
23, below and perpendicular to the slot 29. The single excitation
port is electromagnetically coupled to both the slot antenna 20 and
the antenna element 22. An RF signal injected into the excitation
port 33 propagates along the microstrip feed-line 34, and
electromagnetically couples to the slot 29, producing different
potentials across the slot as represented by a positive charge 31
and a negative charge 32. Consequently, electric fields are
established and distributed exponentially decreasing along the slot
29 with maximum amplitude at the open-end and substantially zero
amplitude at the closed-end. The single excitation port serves to
feed both the slot antenna and antenna element, which is cascaded
with the conductive strip defining the slot antenna.
The potential difference across the slot 29 is further maximized by
the fact that the electrical length 28 of the conductive strip 21
is about a quarter-wavelength at the first frequency, producing
effective radiation from the slot antenna. This differential
potential induces RF currents flowing on the conductive strip 21.
Maximum current is at the ground connection end and minimum current
is at the virtual feed point 30, i.e. a virtual open circuit. The
virtual open circuit provides substantially no electrical
connection with the coil antenna element 22 at the slot resonant
frequency.
At the second frequency, which is lower than the first frequency,
the conductive strip 21 is no longer a quarter-wavelength long but
rather a short distance from the ground connection end. Relatively
strong current is present at the virtual feed point 30 and
effectively becomes the current source to drive the antenna element
22. The electrical length of the antenna element 22 is optimized
(by adjusting the number of turns for example) to achieve resonance
at the second frequency. Note that radiation from the slot 29 is
minimum at the second frequency since there is small difference in
the potentials across the slot. Also note that the conductive strip
21 becomes part of the antenna element 22, contributed to the
electrical length of this antenna at the second frequency. Maximum
currents are at the ground connection end and somewhere in the
middle of the coil depend on its length, the radiotelephone
structure and surrounding environments. As a result, the single
excitation port 33 feeds both the slot antenna 20 and the coil
antenna 22. In addition, at some frequencies in between the first
and the second frequencies, radiations from the slot and the coil
antennas add constructively, producing multi-band operations.
Preferably, the microstrip line includes a tuning portion 35
extending parallel to the long axis of the slot 29 of the slot
antenna 20 to parasitically load the slot. The parallel tuning
portion 35 of the microstrip transmission line is used to
capacitively or inductively load the slot at frequencies to change
the operational band characteristics of the antenna.
In general, the overall length and width of the invented antenna
are limited by the radiotelephone structure and form factor. The
length 28 of the U-shape conductive strip 21 is preferably a
quarter-wavelength long at the slot resonant frequency. The
accumulated length (or equivalently the number of turns) of the
coil antenna determines the second resonant frequency. The
parameters remaining for tuning to achieve optimum efficiency,
bandwidth and input impedance are: a) the width of the slot 29, b)
the distance from the microstrip feed-line portion 34 to the slot
closed-end, c) the extended parallel portion 35 of the microstrip
feed-line, and d) the material properties such as dielectric
constant, loss tangent, and dielectric thickness. These parameters
can be prioritized as follows: parameters a) and b) are the most
sensitive tuning parameters for achieving bandwidth and impedance;
parameter c) is for fine-tuning, and parameter d) has the least
impact. In practice, there are no specific rules for tuning
electrically small antennas mounting in radiotelephones since these
antennas are operating in a continuously changing environments
(laying on the table, holding in hands next to head, being kept in
purse or pocket, etc.) as contrary to electrically large antennas
mounting in a fixed position (on top of a tower or roof). Antenna
behaviors change drastically when covered by hands or held next to
head as in the talking position. Therefore, it should be recognized
that antennas cannot be tuned to satisfy all positions.
In practice, the slot 29 shown in this drawing has a width of
approximately 2 mm and a length of 15 mm. The width of the
conductive strip 21 is 4 mm, uniformly. The microstrip feed-line
(portions 34 and 35) is 1.5 mm wide, and position about 9 mm from
the closed-end of the slot. Tuning portion 35 of the microstrip
feed-line is tunable, typically 12 mm long. Since the feed-line is
short, its width is not sensitive to the antenna performance.
Again, the length (or the number of turns) of the coil antenna 22
is adjusted for resonance at the low frequency band (the second
frequency). The overall dimensions of the invented antenna are 33
mm long and 10 mm wide. Note that the above given dimensions are
for reference. Depending on the phone structure and form-factor,
these dimensions can be changed accordingly to improve
performances. The dielectric used in the present invention is
RO3003 material, which has a dielectric constant of 3.0, 0.5 mm
thick and 1 oz copper. Choosing higher dielectric constant material
will reduce the physical size of the antenna but increase the
loss.
The open-ended slot antenna is configured to operate at the higher
frequency bands including GPS (1500 MHz), DCS (1800 MHz), PCS (1900
MHz) and Bluetooth.TM. (2400 MHz). The coil antenna is configured
to achieve radiation at the lower frequency bands including analog,
CDMA or TDMA (800 MHz) or GSM (900 MHz). In addition, although the
drawings show coils with windings having only a few turns, as many
turns as needed can be easily implemented in the present invention.
Also, the coil can be replaced by microstrip meander-line at the
expense of the size.
FIG. 4 shows a second preferred embodiment of the present
invention. In this embodiment, the slot antenna 20 is identical to
that of FIGS. 2 and 3, and better demonstrates that relative
positions of the microstrip feed-line 33 and the slot 29. In
particular, the parallel portion 35 of the microstrip feed-line 33
is located alongside the slot 29 but not underlying it. The slot
antenna 20 operates identically to that of FIGS. 2 and 3. However,
the coil portion 36 of the antenna element 22 is oriented at
ninety-degrees from that shown in FIGS. 2 and 3. This orientation
gives the option for further mitigation of any cross coupling
between the slot antenna 20 and the antenna element 22. It should
be recognized that this orientation takes advantage of the use of
vias 26, since wraparound conductive traces cannot be used on one
side of the coil. FIG. 5 gives a cross-sectional side view of the
antenna apparatus of FIG. 4 to show a clearer view of the vias
26.
Other changes to the present invention can be made such as adding
additional conductors disposed on the bottom surface of the
dielectric material, with the additional conductors being coupled
across the slot to cause the antenna to be radiant at more
frequency bands. However, multiple conductor configurations must
take into account the interactions between the individual
conductors as well as further possible excitation driven ports. In
addition, the microstrip feed-line can be located closer to the
closed-end of the slot with the parallel portion 35 for tuning
directed towards the open-end of the slot. Along these same lines,
the microstrip feed-line portions 34 and 35 can be reshaped to form
a C-section or a T-section instead of an L-section as shown, as
long as at least one part of the feed-line extended across the
slot, and the tuning portion extends at least partially parallel to
the long axis of the slot. The microstrip feed-line can have other
configurations, such as a curve, however the L-shape is preferred
to reduce the needed surface area for the antenna. Forming the
shape of the microstrip feed-line to a "L", "C", or "T" section, or
any other shape is effectively adding capacitive and/or inductive
shunt components to achieve the desired impedances without adding
external matching network. Further, the antenna apparatus can be
configured such that the second radiant frequency band can be
either higher or lower than the first radiant frequency band.
Although, size constraints limit the preferred embodiment to have
the first (slot) frequency be higher than the second (coil)
frequency.
In the examples shown above, a multi-band antenna apparatus is
shown with two very different types of antenna elements driven by a
single excitation port, yet the two elements radiate at different
frequency bands. Test results have shown that the antenna apparatus
of the present invention provide similar radiation efficiency as an
extended external antenna, and better efficiency than a "stubby"
antenna. This is provided at a low cost and is implemented in a
convenient form factor being located completely internal to a
radiotelephone.
While specific components and functions of the multi-band slot
antenna are described above, fewer or additional functions could be
employed by one skilled in the art within the broad scope of the
present invention. The invention should be limited only by the
appended claims.
* * * * *